1. Field of the Invention
The present invention relates to an apparatus for processing a substrate in an atmospheric gas while controlling a substrate temperature and, more particularly, to an apparatus for processing a semiconductor wafer in the manufacturing process of a semiconductor device.
2. Description of the Related Art
Known as this type of processing apparatus are a sputtering apparatus and so on which are employed, for example, in the manufacturing process of a semiconductor device.
In the sputtering apparatus, in order to fully reduce the concentration of a remaining gas in a vacuum chamber, the interior of the vacuum chamber is set to a predetermined pressure by a vacuum pump with an exhaust conductance set at a maximal value through valve adjustment. Then with the exhaust conductance set to a smaller value, a sputter gas, such as an argon gas, is introduced into the vacuum chamber to reach a predetermined gas pressure. The sputtering gas is changed into a plasma in the neighborhood of a target to allow positive ions in the plasma to collide with the negative voltage-applied target. Through the collision, atoms, molecules, and so on are emitted from the target and the emitted atoms reach an opposed semiconductor wafer surface where a thin film is formed on the surface of the semiconductor wafer.
FIG. 1 is a cross sectional view diagrammatically showing one form of a susceptor of a conventional sputtering apparatus.
In FIG. 1, a semiconductor wafer 1 to be treated is placed on the surface of the susceptor 50 which is situated in a vacuum chamber (not shown) serving as a process chamber. The semiconductor wafer 1 is clamped by a clamp means on the surface of the susceptor. A thin film is formed, by sputtering, on the surface of the wafer 1 which is opposed to the target 2.
A negative voltage is applied to the target 2 placed on a target electrode 62 which is situated in an opposed relation to the semiconductor wafer. A sputtering gas (atmospheric gas) introduced into the vacuum chamber is changed into a plasma to allow those positive ions in the plasma to collide with the target.
In order to obtain a thin film of excellent characteristic on the wafer surface, control has to be made by heating the wafer so that a uniform film may be formed at a predetermined temperature. In the formation of a thin Al film on the wafer 1, for example it is usually necessary to set the wafer temperature to about 200.degree. C. A heater 60 for heating the susceptor 50 is, therefore, provided in the sputtering apparatus. Since the wafer 1 usually has a fine, uneven pattern on its reverse surface, air voids are formed between the wafer 1 and the susceptor 50. The sputtering process is carried out in a vacuum chamber, the air voids are also placed under a vacuum condition. The temperature of the wafer 1 is difficult to control due to a fall in the heat exchange percentage between the wafer and the susceptor. A gas of a predetermined pressure is introduced at an area between the surface of the susceptor and the wafer through a inlet 58 provided at the center of the susceptor 50. In this case, the introduced gas serves as a heat-medium, heating the wafer at that area. One form of this technique is disclosed in U.S. Pat. No. 4,512,391 to Harra.
In order to accurately control the temperature of the wafer, it is necessary that the gas (hereinafter referred to as a heat-medium gas) in the air voids or space 56 between the surface of the susceptor 50 and the wafer 1 be maintained to a predetermined pressure so as to obtain a desired heat exchange characteristics.
In this case, it is unavoidable that the heat-medium gas filled in the space will leak into the vacuum chamber through gaps created between the wafer and the susceptor at the edge of the wafer. Conventionally, in order to maintain a predetermined gas pressure, an amount of leakage involved is refurnished. As the heat-medium gas, the use is made of an inert gas, such as an Ar gas, thereby minimizing a possible bad effect exerted on the process conditions even when a leakage occurs in the vacuum chamber.
With a recent high density integration of the semiconductor elements, it is necessary to enhance the characteristic of a thin film formed by the sputtering apparatus as well as the accuracy with which the film is formed. For this reason, an apparatus has been developed whereby sputtering is done with the use of a sputtering gas of a lower pressure.
However, the conventional sputtering apparatus has such a problem that, due to the external leakage of the heat-medium gas as set out above, the pressure of the sputtering gas cannot be maintained under desired low pressure process condition. The heat-medium gas pressure acting on the rear surface of the wafer is meaningless unless it is set to a certain level adequate to obtain the heat exchange characteristic. Therefore there is naturally a lower limit on the pressure level, even if it is desirable to use a still lower level from the standpoint of reducing a leakage into the vacuum chamber. On the other hand, if the pressure in the vacuum chamber is lower as required, a greater pressure difference is involved between the pressure in the vacuum chamber and the heat-medium gas pressure so that more heat-medium gas leaks into the vacuum chamber from the rear surface side of the wafer. Even where the pressure in the vacuum chamber is controlled to a low level, it is not possible to maintain it under desired low pressure process conditions due to a flow of the heat-medium gas therein.
For example, if a titanium nitride film is to be formed, an Ar/N.sub.2 mixed gas is employed at a predetermined mined partial pressure ratio. Where a plurality of gases are mixed as a sputtering gas, a variation in the partial pressure ratio of a sputtering gas by the flow of the heat-medium gas therein cannot be disregarded as the pressure condition becomes lower.
Further, the heat-medium gas flowing into the vacuum chamber serves as a carrier gas flying about as a dust in the vacuum chamber, causing a fall in the yield of a product due to the contamination of the wafer 1.
Where the wafer is clamped on the susceptor 50 by, for example, a mechanical clamp, etc., having a spring mechanism, if a pressure acting on the rear surface of the wafer varies due to a leakage of the heat-medium gas, the wafer is oscillated due to a variation in the pressure of the gas and in the spring pressure. The oscillation of the wafer facilitates an increase in the leakage of the heat-medium gas on the rear side of the wafer. In this case, the heat-medium gas is sequentially refurnished so as to compensate for such a leakage. Through the repetition of the gas leakage and refurnishing of the gas, the heat-medium gas is pulsated with time, providing a hitch to performing a temperature control of the wafer with high accuracy.
Further, where a sputtering is carried out with the use of a reaction gas and the sputtering gas, the reaction gas, together with the sputtering gas is introduced into the vacuum chamber. The molecules in the reaction gas react with atoms released from the target upon sputtering so that a thin film is formed, as a reaction product, on the semiconductor wafer.
With the target formed by, for example, a titanium (Ti), it is possible to form a titanium nitride thin film over the semiconductor wafer with the use of a nitrogen (N.sub.2) gas as a reaction gas.
With the use of the conventional sputtering apparatus, it is possible to continuously form, on the wafer, a thin film resulting from the reaction of those atoms/molecules released from the target as well as a thin film resulting from the reaction, with a reaction gas, of those atoms/molecules released from the target.
Where, for example a titanium thin film and titanium nitride thin film are continuously to be formed over the semiconductor wafer, then sputtering is conducted under an argon gas atmosphere with a titanium as a target to form a titanium thin film. Then sputtering is carried out through the introduction of the argon gas and nitrogen gas to form a titanium nitride thin film.
The titanium thin film and titanium nitride thin film over the semiconductor wafer are used as a barrier layer when aluminum interconnect lines over the semiconductor wafer are connected to corresponding silicon contacts. When an aluminum layer is formed directly on a silicon layer in an attempt to connect those aluminum interconnect layers to the corresponding silicon contacts, the silicon is precipitated in the aluminum interconnect layer, failing to achieve an adequate electrical connection. In order to avoid such an inconvenience, a titanium thin film is first formed as an intimate layer between the silicon and titanium nitride thin films, then a titanium nitride thin film is formed as a barrier layer over the titanium thin film, and then an aluminum layer is formed over the titanium nitride thin film.
Where, subsequent to continuously forming a titanium thin film and titanium nitride thin film over the semiconductor wafer, a titanium thin film is to be formed over the semiconductor wafer, it is necessary to, after the interior of the vacuum chamber has been subjected to a vacuum to reach a base pressure in the vacuum chamber, again introduce an argon gas in the chamber so as not to allow a nitrogen gas to remain in the vacuum chamber.
However, it takes long to subject the vacuum chamber to a vacuum and hence it takes a longer period of time to achieve a gas exchange and hence to form a thin film over the semiconductor wafer.
With a recent high integration density of a semiconductor element, there is a growing demand that the characteristic of a thin film formed by the sputtering apparatus, as well as the accuracy with which it is formed thereby, be improved. Therefore, there is sometimes the case where the sputtering is done under a lower gas pressure. When a titanium thin film and titanium nitride thin film are to be formed as a barrier layer upon the connection of the aluminum interconnect layers to the silicon contacts as set out above, it is required that the resistivity of the barrier layer be minimized. It is desired that, because the resistivity of the titanium nitride thin film is proportional to a gas pressure upon the formation of a thin film, control be so made as to obtain a minimum gas pressure within a range in which it is possible to obtain a titanium nitride. Where sputtering is conducted under a lower gas pressure, it is necessary that the level of a vacuum be made very high for a base pressure prior to the introduction of the gas. It, therefore, takes an increasingly longer period of time to form a vacuum.
In the conventional sputtering apparatus, a sputter material is deposited on the area beyond means for restricting the direction in which sputter particles fly about, that it, on the inner wall of the vacuum chamber. It has, therefore, sometimes not been possible to obtain an adequate degree of vacuum even if a vacuum is drawn to a high level in the vacuum chamber.
Where a thin film is to be formed by sputtering over the semiconductor wafer, those atoms and molecules released from the target are deposited not only on the wafer but also on the inner wall of the vaccum chamber. The film thus formed on the inner surface of the vacuum chamber has normally an uneven surface, absorbing a greater amount of gas in the vacuum chamber. The gas thus absorbed in the film is released little by little from the film surface upon forming a vacuum in the vacuum chamber, thus lowering the level of a vacuum in the vacuum chamber due to the presence of a resultant gas involved. If the interior of the vacuum chamber is formed to a high vacuum level, an extremely long period of time is required or it is virtually impossible to obtain an adequate degree of vacuum. In this condition, it may not be possible to perform sputtering at such a low vacuum level. Therefore it may not be possible to obtain a thin film of adequate characteristics.